The invention relates to a deflection unit for a color television display tube having a neck portion, a display screen, and a flared outer surface portion therebetween, said deflection unit comprising a field deflection coil and a line deflection coil each formed by a pair of diametrically oppositely positioned coil portions, and an annular core of a magnetically permeable material surrounding at least the line deflection coil, each line deflection coil portion being in the form of a saddle coil and having conductors wound to produce first and second side members, a front end and a rear end which together define a window, with the front end forming a flange, the front end of the coil portions of said line deflection coil, when said deflection unit is mounted on a display tube, being closer to the display screen than are the rear ends, with said front ends substantially surrounding a part of the flared portion of the display tube and the flanges, lying at an angle to the longitudinal axis of said display tube.
Such a deflection unit is commonly used for deflecting the electron beams in color television display tubes. In this known unit, the two coil portions which form the field deflection coil and the two coil portions which form the line deflection coil are both adapted, as regards their shape, to the flared profile of the display tube for which the deflection unit is destined. This means that the individual conductors of the coils engage the glass of the display tube as closely as possible when the deflection unit is mounted on the display tube for which it is intended. This applies in particular to the line deflection coil, since the sensitivity of the line deflection system is an important parameter with respect to the quality of a deflection device. For that purpose it is usual to make the front ends of the coil portions of the line deflection coil arc-like in shape such that they closely follow the contour of the display tube at its flared portion. This contour is often rotationally symmetrical so that the front ends in that case are of circular shape.
More rectangular shapes of this contour are also known, involving a corresponding shape for the front end so that in that case also they optimally conform to the contour of the display tube.
Parameters, known so far which are suitable to spatially shape the magnetic field of a deflection coil of the saddle type and which fully satisfy the requirements with respect to an optimum sensitivity, are provided by the wire distribution of notably the two substantially axially extending parts of each coil portion of which parts the front end forms the connection. Known techniques for this purpose are profiling of the space in the winding mould, profiling of the press die and the insertion of pins in the mould during the winding process. Furthermore it is known that the shape of the soft-magnetic core may also be used as a parameter to some extent.
It is known that in general a color television display system may present errors which may be distinguished as coma, astigmatism, raster defects and linearity defects. For so-called "three in-line guns" display systems it has proved generally possible, by using the above-mentioned design parameters, to make deflection coils by which astigmatism defects are sufficiently minimised.
Coma can also be minimised often in a corresponding manner. The situation is different for the raster defects and the linearity defects. The raster defects are divided into the North-South and the East-West defects. In "in-line" systems the North-South raster defect produces horizontal lines at the lower and upper edges of the picture which show a slight undulating distortion, while the East-West raster defect produces a strong-pin-cushion-like distortion which may be typically between 8 and 14%. Corrections for raster defects and linearity defects are obtained in general by suitable modulations of the line and field deflection currents. In addition, static magnets may alternatively be used for the correction of the undulating distortion.
A known disadvantage of modulating deflection currents, however, is that complicated electronic deflection circuits are required, which moreover consume additional energy and hence provide an expensive solution. In addition to a higher cost-price, the disadvantage of the use of static correction magnets is that, when the correction has to be larger than a few mm, problems arise with regard to the color purity.